Cardiac assist systems aid patients with chronically and unacceptably low cardiac output who cannot have their cardiac output raised to acceptable levels by traditional treatments, such as drug therapy. One particular type of cardiac assist system currently used is a cardiomyoplasty.
Essentially a cardiomyoplasty provides a muscle-powered cardiac assist system. As seen in U.S. Pat. No. 4,813,952 of Khalafalla, incorporated herein by reference, the cardiomyoplasty is a cardiac assist system powered by a surgically-modified muscle tissue, such as the latissimus dorsi. In particular, the latissimus dorsi is wrapped around the heart. An implantable pulse generator is provided. The implantable pulse generator senses contractions of the heart via one or more sensing leads and stimulates the appropriate nerves of the muscle tissue with burst signals to cause the muscle tissue to contract in synchrony with the heart. As a result, the heart is assisted in its contractions, thereby raising the stroke volume and thus cardiac output. Besides delivering therapeutic electrical pulses to the muscle, the pulse generator is quite often also coupled so as to also provide therapeutic electrical pulses to the heart. See, for example, U.S. Pat. No. 4,735,205 of Chachques et al., incorporated herein by reference.
Patients with chronic cardiac output deficiencies, although treatable through cardiomyoplasty, face an increased risk for cardiac arrhythmic episodes, such as ventricular tachycardia or fibrillation. These arrhythmic episodes may be life-threatening.
In order to treat these potentially life-threatening cardiac arrhythmias, some cardiac assist systems have been proposed which combine both a muscle stimulator as well as a cardiac pacer-cardioverter-defibrillator. In such a manner a patient who has had a cardiomyoplasty may, in addition to receiving muscle-powered cardiac assistance, also receive various types of therapeutic cardiac electrical stimulation. One example of such a system may be seen in the U.S. Pat. No. 5,251,621 issued to Collins and entitled "Arrhythmia Control Pacer Using Skeletal Muscle Cardiac Graft Stimulation." In particular the device of Collins provides muscle stimulation while the arrythmia is being confirmed and also immediately prior to the delivery of a defibrillation shock.
One problem associated with devices which combine both a muscle stimulator as well as a cardiac pacer-cardioverter-defibrillator, however, is the extra power requirements of simultaneously delivering skeletal muscle stimulation along with charging a defibrillation output capacitor.
In a new device, this extra power requirement only causes a slightly longer period required for the charging of the defibrillation output capacitor. In general this is an acceptable side effect since the benefits of cardiac augmentation during fibrillation outweigh the increase time required for the delivery of therapy. In particular, the increased cardiac perfusion from the skeletal muscle stimulation lowers the defibrillation threshold.
In devices having less than a full battery, however, the competing demands of concurrent skeletal muscle stimulation along with charging of the defibrillation output capacitor may have more serious consequences. In particular, the battery voltage could collapse and drop below the power-on-reset (hereafter "POR") threshold. If that were to occur the device would reset itself. Functionality would be temporarily lost and all ongoing therapies interrupted or aborted. The result would be any defibrillation therapy would not be delivered until a significant amount of time had elapsed.
Previously, others have attempted to provide some means of protecting against excessively low voltages within a device. U.S. Pat. No. 4,868,908 to Pless entitled "Power Supply Down-Conversion, Regulation and Low Battery Detection System" discloses a low battery detect circuit which shuts down high current circuitry in the system in the event that battery voltage drops to a level such that the regulated output voltage is endangered. Of course, in devices featuring concurrent skeletal muscle stimulation along with charging of the higher voltage, higher current defibrillation output capacitor, shutting down the higher voltage, higher current circuitry is precisely the worst possible thing to do. During VF, the first priority is the delivery of a defibrillation shock, as such the charging of the higher voltage, higher current defibrillation output capacitor should take precedence over the lower voltage, lower current muscle stimulation therapies.